1. Field of the Invention
The present invention relates to the art of injection molding, and especially to the production of prototype parts in the development of new product designs.
2. Prior Art
In the design and development of new plastic products there is often a need to produce a prototype of a part intended eventually to be mass produced by injection molding. The molds typically used in injection molding machines are machined from steel, and this is a slow and expensive procedure if it is intended to produce only a few parts to test a design.
In recent years, one method for making prototype parts cheaply and quickly has been to firstly produce a geometric computer model of the part using computer aided design (CAD). This gives the designer the shape and style of the part. The designer can then go further and use recently developed methods, termed "rapid prototyping systems" or RP systems to convert the geometric computer model into a prototype part. Such rapid prototyping systems include, for example, a so-called "stereolithographic process" using "stereolithographic apparatus" (SLA), or "selective laser sintering", either of which may be referred to as "solid freeform fabrication" (SFF). Various other RP systems have recently become available, and are currently becoming available.
In the SLA system, plastic prototype parts are created from a vat of liquid photocurable polymer by selectively solidifying it with a scanning laser beam. One form of stereolithographic apparatus, or SLA, uses a photosensitive liquid polymer, an x-y scanning ultra-violet laser beam with a 0.25 mm beam diameter, and a z-axis elevator in the vat. The laser beam is focussed on the liquid's surface and cures the polymer, making solid forms wherever the beam is scanned. The depth of the cure is dosage-dependent. The elevator platform is initially positioned at the surface, and as the laser draws a cross-section in the x-y plane, a solid layer is formed on the elevator platform. The elevator is then lowered, and another solid layer is formed on the first. This process is repeated to produce a solid object rising from the elevator platform. To save time, it may be preferred not to fully cure each layer, but to fully cure only the outline of each cross-section and the whole of the bottom and top layers; the internal volume can be cured subsequently under ultra-violet light.
While this SLA system gives an economical way of producing a plastic prototype of a part, such parts are fragile due to the nature of the plastic material, and cannot be tested in an apparatus as though they were functional parts.
It has been proposed in accordance with U.S. Pat. No. 5,439,622 to Pennisi et al. that, instead of using SLA to form the prototype part, this apparatus be used to form mold portions which can then produce prototype parts. In the Pennisi process, the SLA is used to provide a shell defining the inner mold surface, the shell being formed of resinous material of 0.5 to 1.5 mm thickness, and which has some degree of resilience so as to resume its shape after temporary deformation. This shell provides one of the inner faces of the mold portion. It is connected to a "shell base" which forms a cavity with the shell, and this cavity is filled with incompressible material which is said to make the mold portion rigid enough to withstand the forces associated with injection molding. The incompressible material may be high packing density ceramic particles such as silica, alumina, or aluminum nitride. Also, liquids such as water or oil may be used. The cavity is closed by a plug which retains the incompressible material. The shell is coated with a thermally conductive material such as copper or the like, the coating being done by an electroless plating technique, to produce a thin layer of metal about 1-2 .mu.m thick. The metal coating is intended to dissipate heat, which aids in accelerating the molding process.
Two such mold portions are fitted with shoes and placed in an injection molding machine. The mold portions can be used to make plastic bodies having shapes which would prevent their removal from a conventional mold, since such parts can be removed from the Pennisi molds by removing some of the incompressible material to make the mold portions sufficiently flexible that such parts can be removed. Clearly, therefore, the metal coating is intended to be flexible; its purpose is to provide conductivity, and not strength. Also, the incompressible material is intended to be removed and does not solidify.
The Pennisi system has the drawback that, with only a thin layer of metal coating, of 1 to 2 .mu.m thickness, only a few parts can be molded from the from the mold portions before they seriously deteriorate. There is a need for mold parts which are more robust.